Field of the Invention
This invention relates to a method for the production of a metal bearing layer on a cylinder barrel of a hydrostatic displacement machine, in particular of an axial piston machine, in which the metal bearing layer is produced from a sintering powder in a sintering process.
Description of Related Art
On hydrostatic displacement units, such as axial piston machines or radial piston machines, a sliding contact bearing area is located in the reversing mechanism in the vicinity of a control surface between the cylinder barrel and a housing-side manifold. On displacement units of this type, there is a relative movement between the cylinder barrel (provided with the pistons and the displacement chambers) and the housing-side manifold (provided with a low-pressure connection and a high-pressure connection). The connection of the piston bores, and thus the connection of the displacement chambers in the cylinder barrel, with the low-pressure connection and the high-pressure connection in the housing-side manifold is reversed as the piston movement proceeds through the dead center points. To achieve favorable sliding properties and low wear between the cylinder barrel and the housing-side manifold at this sliding contact bearing area, this reversal is realized by a material pairing of a tribologically advantageous layer with a counter-rotation hearing surface made of steel or a cast material. It is known that a bearing metal material can be deposited on the end surface of the cylinder barrel to create a tribologically advantageous, e.g., a low friction, layer.
In known hydrostatic displacement units, the metal bearing layer on a sliding contact bearing component formed by a steel body of the corresponding sliding contact bearing area is applied by a casting process or by a sintering process.
From DE 24 31 254 A1, DE 10 2008 027 698 A1, and DE 10 2008 027 700 A1, it is known that the metal bearing layer can be deposited on the end surface of the cylinder barrel of a hydrostatic displacement machine by a sintering process. A mold element in the form of a sintering ring is placed on the cylinder barrel and a sintering powder is filled into the cavity created by the sintering ring. Two successive filling processes with sintered powder can be carried out to improve the properties of the metal bearing layer by means of a two-layer charge of packed powder. The packed powder charge is then compacted in a compacting machine and sintered. These processes can be carried out simultaneously by performing the sintering with the simultaneous application of pressure and heating to the sintering temperature in a pressure sintering device. This process is also called pressure sintering.
With a manufacturing method of this type, however, the uniformity of the filling of the cavity created by the sintering ring placed over the cylinder barrel fluctuates a great deal. This results in a dispersion of the physical properties of the metal bearing layer on the cylinder barrel.
During pressure sintering, the force with which the sintering powder charge is compacted by the pressure sintering device is also limited.
A great deal of dust is also generated during the compacting of the sintering powder charge applied to the cylinder barrel. To reduce the amount of dust generated, ethanol or glycol as additives can be added to the sintering powder. However, the addition of ethanol or glycol makes the sintering powder more expensive. The large amount of dust generated also results in a loss of sintering powder represented by the dust, which further increases production costs. The large amount of dust generated also requires employees to permanently wear protective respiratory equipment. With these additives in the sintering powder, the amount of dust generated during the compacting of the sintering powder charge can be reduced, although the additives ethanol or glycol have additional disadvantages in terms of the coating quality of the sintering powder charge. The layer of powder can be non-uniform, in particular, in the radially outer peripheral areas and, thus, in the load-bearing web area of the cylinder barrel. The additives ethanol or glycol in the sintering powder on one hand interfere with the pourability of the sintering powder during the process of filling the sintering ring and, on the other hand, evaporate due to high temperatures during the sintering process. As a result, small cavities are formed in the metal bearing layer. When the sintering powder is a bronze powder that contains lead, e.g., a mixture of copper, tin, and lead, these cavities are filled with lead during the sintering process because lead is not fully incorporated into the alloy and is present as a separate liquid phase, causing local decreases in hardness to occur in the sintered metal bearing layer. Overall, with the known pressure sintering processes there can be quality problems with the metal hearing layer applied to the cylinder barrel.
It is an object of this invention to provide a method for producing a metal bearing layer on a cylinder barrel of a hydrostatic displacement machine, in particular of an axial piston machine, with which the metal bearing layer on the cylinder barrel can be produced with high quality, at low production costs, and with a reduced quantity of generated dust.